The present invention relates to telemetry in general, and, more particularly, to a system in which a plurality of transmitters wirelessly transmit data for collection by one or more receivers.
Some wireless telemetry systems (e.g., burglar alarms, fire alarms, power utility meters, leak detectors, environmental monitoring, etc.) comprise many transmitters that periodically or sporadically transmit messages to one or more receivers. In these systems, the transmitters are located at different places and transmit messages that are indicative of the status of monitoring sensors to a receiver that collects the data from all of the sensors. Normally, the transmitters transmit messages that are as short as feasible and with the interval between the transmissions as long as feasible. This is advantageous for two reasons. First, it minimizes the average current drain in the transmitters, which are typically battery operated. Second, short and infrequent transmissions lower the probability that the data is lost due to collisions that occur when two or more transmitters transmit at the same time. However, if an alarm occurs, the associated transmitter transmits immediately in order to convey the alarm message with little delay.
Typically, such systems transmit data at a single frequency, and thus are susceptible to interference and signal loss due to a phenomena known as xe2x80x9cmultipath fading.xe2x80x9d Consequently, the reliability of such systems is compromised or, conversely, the transmitted power has to be increased to overcome the fading which results in larger power drain and shorter battery life. Besides, there usually are regulatory limits that restrict such transmitter power and thus limit the possible compensation by sheer increase of power. Since the multipath effect is highly sensitive to the frequency of the transmitted carrier, a system using multiple frequencies (e.g., a frequency hopping spread spectrum system, etc.) has a potential to eliminate these drawbacks. However, frequency hopping systems require a long acquisition time and they are typically used in two way communication applications in which all the devices are continuously synchronizing with one master device or with each other using a variety of synchronization methods as shown in some references. In other cases, to ease the synchronization problem, there are employed receivers that can simultaneously receive signals at many frequencies by making the receiver broadband or by using several receivers at the same time. Generally, those solutions suffer from performance degradation or high cost or both which makes them undesirable for low cost applications that require high reliability such as security systems and some telemetry systems.
A serious problem that must be addressed in battery operated systems is to shorten the transmission time as much as possible by making the message preamble as short as possible in order to conserve the battery power. Therefore, the synchronization of the receiver with the transmitters is a difficult task. This problem is exacerbated in some systems such as security alarms that require some messages to be conveyed to the system immediately without waiting for the scheduled transmission time. A related problem, in battery operated systems, is limitation of the transmitted power to conserve the battery power.
Some embodiments of the present invention comprise a frequency hopping receiver that acquires and maintains synchronization with a plurality of transmitters, which enables the transmitters to omit the transmission of long preambles. This is advantageous because it lowers the average current drain in the transmitters and, consequently, lengthens their battery life. Furthermore, some embodiments of the present invention are advantageous in that they provide improved reliability in the presence of multipath fading, interference and jamming. And still furthermore, some embodiments of the present invention are capable of eliminating the effect of persistent collisions that occur when two or more transmitters transmit at the same time in the same channel for a prolonged period.
The illustrative embodiment of the present invention is a frequency-hopping wireless telemetry system comprising: (1) one or more receivers, and (2) one or more transmitters, each of which receive input from one or more sensors. The transmitters intermittently transmit very short messages indicative of status of the sensors associated with the transmitters. Each transmitter includes a time interval generator to establish the time interval between successive transmissions, a frequency synthesizer-modulator to generate a modulated radio frequency carrier signal wherein the frequency of the carrier changes in response to programming the synthesizer by digital data, a reference frequency oscillator to provide a frequency reference from which the synthesizer derives carrier frequencies and, advantageously, from which the time interval generator derives its timing, and a transmitter control logic activated in response to pulses from the time interval generator or a sensor signal indicating an abnormal condition. When activated, the transmitter control logic activates and programs the synthesizer so that the transmitter carrier frequency is changed according to a frequency hopping algorithm, provides digital data indicative of the sensor status and advantageously battery status, and modulates the carrier with the provided data. The receiver includes a frequency selective radio receiver circuit, programmable by digital data, to receive and demodulate a transmitted carrier when the frequency of the receiver circuit is programmed according to the frequency of the carrier, and a receiver control logic means to process demodulated data, to provide system interface responsive to the received messages, and to program the frequency of the frequency selective receiver circuit. The control logic includes a receiver timer to measure the elapsing time, and a plurality of memory registers to hold digital data indicative of (a) the time of the next transmission occurrence for each transmitter and (b) the frequency of the next transmission occurrence for each transmitter. In operation, the control logic sequentially compares the data content of the time registers with the data content of the timer and if the transmission is due from a transmitter, the control logic programs the frequency selective radio receiver circuit according to the data content in the frequency register associated with said transmitter, attempts to decode the demodulated signal, modifies the content of the time register by a number representative of the time interval between the successive transmissions for said transmitter and modifies the content of the frequency register according to a predetermined algorithm for said transmitter.
In accordance with the illustrative embodiment of the present invention there is provided a method of transmission in the system so as to improve reliability of the system in the presence of multipath fading and interference, the method is based on varying the transmission frequency for each transmitted message and varying the time between consecutive messages. The frequency variations provide frequency diversity and are effective against multipath fading as well as single of multiple narrowband interference. The time variations are effective against periodic impulse interference. In combination, the frequency and time variations provide immunity for a wide variety of signal impairments and interference including multipath fading, wide and narrowband interference, impulse noise and deliberate jamming.
In accordance with the illustrative embodiment of the present invention there is provided a method of minimizing the effect of collisions, the method is based on selecting the transmission frequencies in sequences that are different for each transmitter, wherein transmitter frequency sequence depends on the transmitter ID number or other number derived or associated with the transmitter ID. In addition, in the illustrative embodiment, the transmitter ID number or other number derived or associated with the transmitter number is included in the transmitted message, so that, upon reception of a single message from a transmitter, the receiver can determine what is the next frequency for this transmitter, and thus achieve synchronization with this transmitter.
In accordance with the illustrative embodiment of the present invention there is provided another method of minimizing the effect of collisions that can be used alone or in conjunction with the third aspect of this invention, the method comprising randomizing the time interval between transmissions individually for each transmitter and a receiver compensating for the time interval changes.
In accordance with the illustrative embodiment of the present invention there is provided a simple method to generate a very large number of frequency-time hopping sequences. The method produces sequences that are orthogonal, thus eliminates possibility of persistent collisions even when large number of transmitters are used. In addition, the method requires identical circuit in each transmitter and the actual sequence that is produced is selected by the transmitter ID or other number associated with the transmitter ID, thus making it convenient for manufacturing. Also, the method enables to produce a very large number of frequency-time sequences based on a single short PN generator whose state can be instantly recovered by a receiver based on just one received transmission, thus aiding the receiver in obtaining synchronization with a transmitter whose ID is known. At the same time, because of a very large number of possible sequences that can be generated, it is difficult to obtain synchronization if the transmitter ID is not known, which makes the system immune to interception and jamming.
In accordance with the illustrative embodiment of the present invention there is provided a method that enables such a system to convey the information about an abnormal sensor condition as soon as the condition occurs regardless of the transmission period of the associated transmitter. The method is based on selecting an alarm frequency or advantageously a group of alarm frequencies common for all transmitters. The alarm frequencies are used by the transmitters when an alarm or an abnormal sensor condition occurs or on power-up, wherein when such a condition occurs in a transmitter, the transmitter transmits the messages sequentially on the alarm frequencies for a predetermined period of time after which the transmitter resumes transmissions according to the sequence before the alarm condition, wherein the receiver monitors the alarm frequencies during the time between the reception of scheduled messages from the transmitters.